Interpretive Summary: Casein precipitated with high pressure carbon dioxide (CO2-casein) has physical properties that make it a desirable biodegradable ingredient for use in films or coatings. Films made with CO2-casein and water with glycerol added to make the films more flexible are virtually insoluble in water and are stronger and stiffer than films made from casein precipitated the conventional way. It is relatively easy to make films in the laboratory by pouring a solution of casein, glycerol and water into a dish and allowing it to dry undisturbed to properly form a thin film. However, the technology for making these films on a large scale is unknown at this time. A continuous pilot plant process was developed that uses specific belt materials, feed mechanism, and drying times and temperatures to make films from CO2-casein/glycerol/water solutions. It was determined that up to 20% of the casein can be replaced with nonfat dry milk (dried skim milk) with little loss in physical properties thereby reducing the cost of the films. This research developed a process that can successfully make CO2-casein films continuously and established the potential for the commercial production of biodegradable polymer coatings made from dairy products and can be modified for other proteins.

Technical Abstract:
Casein precipitated with high pressure CO2 (CO2-casein) has unique physical properties compared to caseinate. Films made with CO2-casein and plasticized with glycerol are barely soluble in water [calcium (Ca) caseinate films are soluble in water] and CO2-casein films are about 25% stronger and about 15% stiffer than corresponding Ca caseinate films. A continuous pilot plant process was developed to make films from CO2-casein/glycerol solutions. CO2-casein/glycerol solutions wet and spread on polyethylene belts and were easily removed after forming films. To reduce the cost of the films, non-fat dry milk was substituted at various levels up to 50% for Ca caseinate in cast films, and CO2-casein film in the continuous film process. Non-fat dry milk costs less than CO2-casein and 60% less than Ca Caseinate. The films were tested for tensile strength and elongation. There was little loss in physical properties with substitution of non-fat dry milk up to 20%. Thus non-fat dry milk can be substituted up to 20% for either Ca caseinate or CO2-casein in the continuous process. The process that has been developed for CO2-casein films should apply to other film forming proteins.